This invention relates to a diaphragm pump for delivering a pressure fluid to, for instance, a spray gun for spraying a pressure fluid or liquid, such as a paint, onto a wall.
In a diaphragm pump of the type described, there are provided a pump chamber into or out of which a pressure fluid is fed, on one side of a diaphragm and a pressure fluid chamber, into or out of which a pressure fluid adapted to deflect the diaphragm, such as for instance oil, is fed, with the diaphragm positioned between the pump chamber and the pressure fluid chamber, while there are provided for the pump chamber two check valves adapted to open or close alternately, one of the check valves being connected to the pressure fluid reservoir containing a liquid such as a paint, and the other of the check valves being connected via a discharge pipe to a spray gun. In addition, the pressure fluid chamber communicates with a cylinder, in which a plunger reciprocates. The plunger is moved back and forth by means of a cam or the like so as to raise or lower the fluid pressure in the pressure fluid chamber, thereby deflecting the aforesaid diaphragm so as to expand or contract the volume of the pump chamber alternately. At the time of expansion, the paint or the like is drawn by suction through one of the check valves, and at the time of contraction, the paint is supplied via the other check valve to a spray gun or the like.
A relief valve is provided between the pressure fluid chamber or cylinder and the pressure fluid reservoir, thereby preventing an excessive pressure rise in the pressure fluid chamber. For instance, in case a nozzle of the spray gun is closed, the load acting on the pump chamber is increased, so that the diaphragm can no longer be operated or deflected, so the pressure of the fluid in the pressure fluid chamber becomes too high during a compression stroke, in which the plunger is moved towards the diaphragm. However, at this time the pressure fluid is bled via the relief valve into the pressure fluid reservoir, thus ensuring safety of the diaphragm pump. For instance, when the nozzle of a spray gun is opened, with the relief valve being set to a high pressure level, then the pump chamber starts delivering the paint under pressure, while the diaphragm is repeatedly deflected to a large extent proportional to the progress the plunger stroke through the medium of a pressure fluid, in response to the back and forth displacements of the plunger. However, it is rare that the fluid pressure in the pressure fluid chamber exceeds the adjusted pressure level for the relief valve, so that little or no fluid is bled through the relief valve.
However, if the relief valve is set to a relatively low pressure level, then the pressure of the fluid in the pressure fluid chamber necessarily exceeds the adjusted pressure level for the relief valve, in the course of the pressurizing stroke of the plunger, after which the pressure fluid is continuously bled into the pressure fluid reservoir via the relief valve, until the termination of the pressurizing storke. Accordingly, in this case, during the pressure reducing stroke following the pressurizing stroke or during the time from the pressure reducing stroke to the beginning phase of the subsequent pressurizing stroke, an amount of pressure fluid corresponding to the amount of pressure fluid which has been discharged from the cylinder and pressure fluid chamber during the pressurizing stroke should be supplied from the pressure fluid pump to the cylinder and pressure fluid chamber.
To this end, a passage for supplying the pressure fluid is provided between the pressure fluid reservoir and the cylinder or the pressure fluid chamber. However, if the internal pressure is a vacuum, the pressure fluid flows at a high speed through the aforesaid passage, so that the pressure fluid is agitated vigorously, thereby producing heat due to internal friction. The temperature rise stemming from the heat of the pressure fluid reduces the allowable operating speed of the pump, thus leading to deterioration of the pressure fluid. Many attempts to solve this shortcoming have been proposed.
For instance, U.S. Pat. No. 3,254,845 (this will be referred to as the Schloser patent, hereinafter) is intended to prevent a temperature rise by utilizing a cooling effect resulting from a cavitation phenomenon. More particularly, in the suction stroke, when the vacuum in the pressure fluid chamber reaches a given set value below the saturated vapor pressure of the pressure fluid, a fluid regulating valve of a lead type, which is provided in the front of a piston, is opened, so that the pressure fluid flows from the reservoir into the pressure fluid chamber via a passage in the piston and the pressure regulating valve, thereby preventing excessive reduction of the vacuum in the chamber. The pressure regulating valve is opened only during the period of time that a vacuum in the pressure fluid chamber remains below the saturated vapor pressure of the pressure fluid, so that the pressure fluid is continuously vaporized during the pressure reducing stroke, and the pressure fluid goes into vapor form, thereby resulting in the so-called cavitation phenomenon. However, cavitation is responsible for the premature corrosion of cylinders and pistons, and hence is not desirable. It is an object of the diaphragm pump according to the present invention to dispense with any auxiliary cooling means or heat exchanger which have otherwise been required for the prior art pumps of this type, by purposely utilizing the cooling effect stemming from the cavitation phenomenon.
U.S. Pat. No. 3,680,981 (this will be referred to as the Wagner patent, hereinafter) is intended to avoid the premature corrosion of the pump wall due to cavitation caused by the pressure fluid, by agitating the pressure fluid so as to increase the air content of the pressure fluid, thereby preventing the reduction of the internal pressure of the cylinder to below the vaporizing pressure of the pressure fluid, by utilizing air emitted from the pressure fluid in the cylinder. The emission of air from the pressure fluid may be considered to be cavitation in a broad sense. However, this provides a cooling effect to some degree, so that there results no marked damage, as compared with that caused by the cavitation of a pressure fluid. The Wagner patent is directed to avoiding a temperature rise by utilizing the aforesaid cavitation defined in a broad sense, in a range such that the pressure fluid does not cause the cavitation. Accordingly, lower the internal pressure in a cylinder during a pressure-reducing stroke of the plunger, the more preferable, insofar as the pressure fluid does not cause cavitation. For this reason, according to the Wagner patent, an opening on the side of cylinder, to a pressure-fluid-supply or replenishing passage is positioned at the extremity of the pressure reducing stroke of the plunger. However, the supplying of pressure fluid should be accomplished within an extremely short time, while the pump is continued to be operated, irrespective of whether the aforesaid supply is sufficient or not, because the time of the passage remains open is controlled mechanically. In case the amount of the pressure fluid supplied is not sufficient, and the amplitude of the diaphragm moment is reduced, so that the supply of pressure fluid becomes insufficient and unstable.